The development of mobile communications technique not only meets people's needs for voice service but also more and more effectively satisfies the needs for value-added services including the interaction of imaging, audio and real-time multimedia services, consequently the mobile communications system gradually plays a significant role in people's life and work. Among the above, the trunking communication has increasingly become an indispensable service, and the digital trunking system is an important branch of the mobile communication system. The digital trunking system refers to a specific mobile communication system in which several users share a group of wireless channels and dynamically use these channels. It is a command and dispatch system which is urgently needed in the departments having high requirement for command and dispatch functions, such as enterprises, undertakings, industry and mining, oil field, farm, public security, military and police in the countries having relatively developed society and economy, industry and agriculture.
The main characteristic of the trunking service is group call, i.e. after a user inputs the group numbers at the terminal, only by pressing PTT (Push To Talk) key he/she can initiate a group call to communicate with all the group members.
To a terminal of any user, the data transmitted through the forward channels is independently encoded and modulated, but for a group call, the data sent to all members of the group is the same. If the mechanism of sharing the wireless channel is introduced, the data shall be superposed before sent through the forward channels, which means that they can be actually seen as one signal. By doing so, the forward channels of these group members can be considered as being combined into one forward channel for sending. The advantage of the above is that the forward transmission power resource is effectively saved and the system capacity is improved.
However, in the techniques now provided, it can not be guaranteed that the forward channels of all group members under the BSS (Base Station Subsystem) are established at one carrier frequency, therefore the sharing of channel can not be achieved.